Polyolefins such as polyethylene and polypropylene may be prepared by particle form polymerization, also referred to as slurry polymerization. In this technique, feed materials such as diluent, monomer and catalyst are introduced to a reaction zone (for example, a loop reaction zone), and a fluid slurry containing solid polyolefin particles, diluent, and unreacted monomer is circulated through the loop reaction zone.
In continuous loop reactors, the various feed materials may be introduced to the loop reaction zone in various ways. For example, the monomer and catalyst may be mixed with varying amounts of diluent prior to introduction to the loop reaction zone. In the loop reaction zone, the monomer and catalyst become dispersed in the fluid slurry. As the fluid slurry circulates through the loop reaction zone, the monomer reacts at the catalyst in a polymerization reaction. The polymerization reaction yields solid polyolefin particles in the fluid slurry.
Slurry polymerization in a loop reaction zone has proven commercially successful. The slurry polymerization technique has enjoyed international success with billions of pounds of olefin polymers being so produced annually. With this success has come the desirability, in some circumstances, of building a smaller number of large reactors as opposed to a larger number of small reactors for a given plant capacity.
Larger reactors lead to larger flow rates of fluid slurry. The flow rate inside a loop reactor can range typically from 15,000 gallons (56,781 liters) per minute to 1,000,000 gallons (3,785,410 liters) per minute or more.
Conventional polymerization processes may utilize continuous take-off or traditional settling legs for removing fluid slurry from the reactor. In a polymerization process utilizing continuous take-off, the withdrawn slurry may be continuously removed through a take-off appendage.
The withdrawn slurry is usually a small portion of the fluid slurry that is in a loop reaction zone. The flow of this smaller withdrawn slurry typically ranges from 50 gallons (189 liters) per minute to 3000 gallons (11,356 liters) per minute. The large flow in the reactor can transport polymer in the form of slurry particles that are small, for example, a size distribution of slurry particles in which 99% are less than 0.1 inch in nominal diameter, but also larger polymer particles or fused chunks of polymer that can range in size from 0.1 inch up to the internal diameter of the reactor, typically 24 inches or so.
The larger polymer chunks or particles, with diameters larger than the take-off valve control opening, may plug the take-off valve. When such larger particles attempt to pass through the take-off valve, either the particle breaks or the control valve is restricted in flow.
Flow restriction causes loss of flow through the take-off valve and may cause additional build-up of polymer particles, which can lead to plugging. Polymer build-up at the take-off valve causes the reactor pressure to increase, since the reactor pressure is usually controlled at least in part by the take-off valve opening. If the build-up in polymer particles is quicker than the action of the control mechanism for controlling pressure by opening the take-off valve, a plugged line and excessive reactor pressures result. This is especially severe for fused or a typical polymer chunks that can grow in the loop reactor that have a much larger dimension than the largest polymer particle size. Plugged reactor take-off valves can lead to reactor overpressure, downtime, production loss, and in extreme situations, relief of reactor pressure by process safety relief valves.